Road Mortality Relative to Wheel-Track Position during Spring Migration of Common Toads in Norway

Road mortality is a widely recognized, but rarely quantified threat to the viability of amphibian populations. This paper summarizes road mortality during the spring migration for a northern Common Toad (Bufo bufo) population in Norway for 10 of the last 22 years, in addition to assessing where on the road (which wheel-track) toads were most exposed to mortality risk for 2 observation years. Percentage of observed dead toads for all observation years ranged from a mean of 56.9% in 2001 to 83.2% in 2006. The highest annual number of migrating toads was 1,082 in 2006, of which 764 were killed. When adjusting for effective counting days with positive toad activity, average number of toads per day decreased from 40.5 in the years 2001–2007 to 24.2 in 2020–2023, an average decrease of 40%. As a global mean for all 10 counting years, 69.4% of toads were killed on the road. When assigning dead toads to the closest wheel-track, the highest mortality in numbers was found at the start of the road, at wheel-track 1, for both years. Fractional risk of mortality was highest at the fourth wheel-track in 2007, with 47% of toads entering this track being killed, whereas the first wheel-track had the highest fractional death in 2023, with 43% dead animals. Among live migrating animals in 2023, there were 3 females and 56 males, indicating a highly male-dominated migrating population of toads.